Neuromuscular versus mechanical stretch-induced changes in contra- versus ipsilateral muscle

PURPOSE: Whether or not the homologous contralateral muscle (CM) undergoes stretch-induced force reduction as the stretched muscle (SM) is still unclear. The neuromuscular and mechanical factors underlying the force reduction in CM and SM were investigated. METHODS: Twenty-one participants underwent unilateral knee-extensors passive stretching. In both CM and SM, before, immediately after (POST), 5 (POST5) and 10 min (POST10) after passive stretching, maximum voluntary contraction (MVC), peak force (pF) and voluntary activation (VA) were measured. During MVC, the electromyographic and mechanomyographic root mean square (EMG RMS and MMG RMS, respectively) was calculated in rectus femoris, vastus lateralis and vastus medialis, together with M-wave. The total electromechanical delay (EMD), divided in Δt EMG-MMG and Δt MMG-F was calculated. RESULTS: In CM at POST, the decrease in MVC [-11%, CI95% -13 to -9, effect size(ES): -2.27] was accompanied by a fall in VA (-7%, -9 to -4, ES: -2.29), EMG RMS (range -22 to -11%, ES: -3.92 to -2.25), MMG RMS (range -10 to -8%, ES: -0.52 to -0.39) and an increase in Δt EMG-MMG (≈+10%, ES: 0.73 to 0.93). All changes returned to baseline at POST5. In SM, decrease in MVC (-19%, -24 to -18, ES: -3.08), pF (-25%, -28 to -22, ES: -4.90), VA (-10%, -11 to -9, ES: -5.71), EMG RMS (≈-33%, ES: -5.23 to -3.22) and rise in MMG RMS (range +25 to +32%, ES: 4.21 to 4.98) and EMD (≈+28%, ES: 1.59 to 1.77) were observed at POST and persisted at POST10. No change in M-wave occurred. CONCLUSIONS: The contralateral central motor drive stretch-induced inhibition seems to account for the force reduction in CM. In SM, both central inhibition and mechanical factors concurred.